People are saying that this is complicated and you need to input combo's quickly etc but I found a great video on YouTube and all credit goes to the creator. You do need 2 controllers for this method though.
Here's a breakdown of it:
1) Start arcade mode, set the rounds to 2 and the time limit to 10 seconds.
2) On controller 1 choose Candy
3) Press start on controller number 2
4) On controller 2 choose Mahler (from now on always use controller number 2)
5) In the first round hold left to push candy against the wall, now press down and triangle until the time is up. (this weakens her lower armor)
6) In round 2 press up and circle at the same time to kick Candy into the air, now quickly press up and X to and Mahler will jump into the air, land on Candy and destroy her lower armor.
7) Repeat this 10 times in a row for the trophy.
Feel free to comment any suggestions